Graphene is an aromatic conducting polymer comprising a monolayer of sp2-bonded carbon atoms in a planar honeycomb network. Due to its properties of electrical and thermal conductivity, mechanical strength and rigidity, chemical stability and high specific surface area the graphene polymer holds great promise in many technological fields, such as nanoelectronics, sensors, separation/filtration, nanocomposites, batteries, supercapacitors and hydrogen storage. However, an efficient approach to producing processable graphene sheets in large quantities has been a major obstacle to successful commercial development.
Like carbon nanotubes and many other nanomaterials, a key challenge in the synthesis and processing of bulk-quantity graphene sheets is aggregation. In view of their high specific surface area, and unless well separated from each other, graphene sheets tend to form irreversible agglomerates or may even restack to form graphite, as a result of van der Waals interactions. This problem has been encountered in previous efforts aimed at large-scale production of graphene through chemical conversion or thermal expansion/reduction. The prevention of aggregation is of particular importance for graphene sheets because many of their unique properties are only associated with the non-aggregated form of the material.
Aggregation can be reduced by the attachment of other molecules or polymers onto the sheets. However, the presence of foreign stabilisers is undesirable for many applications. In particular, other molecules or polymers can block a portion of surface area of graphene and may decrease electrical conductivity.
Accordingly, there exists a need for a new or improved graphene-based material in which the graphene sheets are separated but stable, so the properties of the individual graphene sheets can be effectively harnessed.
Uniform graphene paper films have been formed on a membrane filter by vacuum filtration of as-reduced dispersions. Fr ee-standing graphene paper can be peeled off from the membrane filter and is bendable with a shiny metallic lustre. The conductivity of graphene paper is found to be about 7200 S/m at room temperature, which is comparable to that of chemically modified single-walled carbon nanotube paper.
Filtration is an efficient technique for making macroscopic assemblies from a suspension of solid particles. Filtration has been widely used for manufacturing writing paper since ancient times and more recently has been used in the fabrication of carbon nanotube and graphene-based papers. Nevertheless, only dried paper products have been targeted by the filtration methods and little attention has been paid to the formation mechanism of graphene paper.
The present inventor has now identified a gel film at the interface of a filter membrane and a liquid dispersion of graphene sheets being filtered. When the liquid is water, the gel film formed is a hydrogel. The inventor has furthermore determined that the gel film produced possesses a number of advantageous and unexpected properties, such as mechanical strength and electrical conductivity. The gel film also has an open pore structure with a highly accessible surface area of individual graphene sheets, which is improved in comparison to the properties of dried graphene paper.
The gel film according to the invention comprises graphene sheets that are arranged in a substantially planar manner, and can be distinguished in this way from known three-dimensional (3D) gels in which the graphene sheets are disposed in random orientation (or plane) relative to one another. Most gels are 3D polymeric networks containing large quantities of a liquid, such as water, but which behave like a solid due to the cross-linked network of polymer within the liquid. Graphene sheets have been reported to form a 3D gel in water if the concentration of a dispersion exceeds a critical value (i.e. 0.5 mg/ml). The resultant 3D hydrogel comprises a highly porous, randomly cross-linked 3D network of graphene sheets. However, the 3D hydrogel formed in solution is fragile, which limits its practical commercial use. A 3D hydrogel has also been reported by Xu et al in a journal article entitled: Self-assembled graphene hydrogel via a one-step hydrothermal process (ACS Nano (2010) 4 (7) pp 4324-4330). The 3D gel was prepared by hydrothermal reduction of a homogenous aqueous dispersion of graphene oxide (0.5 to 2 mg/ml) in an autoclave at 180° C. for 1 to 12 hours. The resultant 3D hydrogel comprises a network of graphene sheets with a poor ordering along their stacking direction. Due to the fact that the sheets are three-dimensionally restricted and that the graphene itself is rigid, the 3D hydrogel is not as mechanical flexible, pH responsive or as conductive as a gel comprising graphene sheets arranged in a substantially planar manner. A gel comprising graphene sheets arranged in a substantially planar manner can also be formed into thinner film than 3D gels which makes them easier to integrate into some devices.